Marry an unemployed girl and have her change CDs
Harddrives as backups have the disadvantage that you have one one or very few generations, and obviously no permanent one. If you have a damaged file due to memory corruption you might find out about it only after you overwrote all generations.
An alternative is a first full backup on a harddrive and then incremental backups on DVDs. That would work well for the backup if you have low change rates, but would be a nightmare to restore. Remember you can
never overwrite the harddrive if you want to avoid problem 1). So if you backup every week you have at least 100 CDs after two years, and you need to red through the harddrive and all 100 DVDs to restore a single file.